Microscopic Anatomy of the Upper Aerodigestive Tract in Harbour Seals (Phoca vitulina): Functional Adaptations to Swallowing.

Abandoned harbour seal pups (Phoca vitulina) are frequently recovered by rehabilitation centres and often require intensive nursing, gavage feeding and swallowing rehabilitation prior to anticipated release. Seal upper aerodigestive tract (UAT) histology descriptions relevant to deglutition are limited, impacting advances in rehabilitation practice. Therefore, we examined the histological characteristics of the harbour seal UAT to understand species-specific functional anatomy and characterize adaptations. To this end, we conducted gross dissections, compiled measurements and reviewed histologic features of the UAT structures of 14 pre-weaned harbour seal pups that died due to natural causes or were humanely euthanized. Representative samples for histologic evaluation included the tongue, salivary glands, epiglottis, and varying levels of the trachea and esophagus. Histologically, there was a prominent muscularis in the tongue with fewer lingual papillae types compared to humans. Abundant submucosal glands were observed in lateral and pharyngeal parts of the tongue and rostral parts of the esophagus. When compared to other mammalian species, there was a disproportionate increase in the amount of striated muscle throughout the length of the esophageal muscularis externa. This may indicate a lesser degree of autonomic control over the esophageal phase of swallowing in harbour seals. Our study represents the first detailed UAT histological descriptions for neonatal harbour seals. Collectively, these findings support specific anatomic and biomechanic adaptations relevant to suckling, prehension and deglutition. This work will inform rehabilitation practices and guide future studies on swallowing physiology in harbour seals with potential applications to other pinniped and otariid species in rehabilitation settings. This article is protected by copyright. All rights reserved.